Turnover of rat liver tyrosine transaminase: stabilization after inhibition of protein synthesis

Turnover of the rat liver tyrosine transaminase in vivo was measured by a label and chase procedure under conditions where the amount of enzyme undergoes no change. Half-life of the (14)C-labeled enzyme in this basal condition was found to be 1.5 +/- 0.3 hours. Inhibitors of protein synthesis (cycloheximide or puromycin) do not appreciably influence the basal enzyme level over a 5-hour period, although these drugs will block hormonal induction of this enzyme. In pulse-labeling experiments, cycloheximide blocked transaminase synthesis almost completely. The conclusion that enzyme degradation, as well as synthesis, must be blocked when protein synthesis is stopped was confirmed in experiments showing that labeled enzyme is stable in the liver of rats treated with cycloheximide The participation of a continuously synthesized polypeptide in the degradative phase of transaminase turnover is suggested.